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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../language/attributes/fallthrough.html below:

C++ attribute: fallthrough (since C++17)

Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.

May only be applied to a null statement to create a fallthrough statement ([[fallthrough]];).

A fallthrough statement may only be used in a switch statement, where the next statement to be executed is a statement with a case or default label for that switch statement. If the fallthrough statement is inside a loop, the next (labeled) statement must be part of the same iteration of that loop.

void f(int n)
{
    void g(), h(), i();
 
    switch (n)
    {
        case 1:
        case 2:
            g();
            [[fallthrough]];
        case 3: // no warning on fallthrough
            h();
        case 4: // compiler may warn on fallthrough
            if (n < 3)
            {
                i();
                [[fallthrough]]; // OK
            }
            else
            {
                return;
            }
        case 5:
            while (false)
            {
                [[fallthrough]]; // ill-formed: next statement is not
                                 //             part of the same iteration
            }
        case 6:
            [[fallthrough]]; // ill-formed, no subsequent case or default label
    }
}

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• C++23 standard (ISO/IEC 14882:2024):

• 9.12.6 Fallthrough attribute [dcl.attr.fallthrough]

• C++20 standard (ISO/IEC 14882:2020):

• 9.12.5 Fallthrough attribute [dcl.attr.fallthrough]

• C++17 standard (ISO/IEC 14882:2017):

• 10.6.5 Fallthrough attribute [dcl.attr.fallthrough]

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